The cryogenic distillation of air for separation into its components is well known. One of the most widely employed cryogenic air separation processes employs the use of a higher pressure column, in which a preliminary separation of air is made into oxygen-richer and nitrogen-richer components, and a lower pressure column, in which the final separation into product oxygen and/or product nitrogen is made. Often the two columns are in heat exchange relation and the lower pressure column is situated over the higher pressure column.
Such double column processes are employed because a single column cannot produce relatively high purities of both oxygen and nitrogen. A second column takes advantage of the shape of the nitrogen-oxygen equilibrium curve so that relatively high purities of both nitrogen and oxygen can be produced. The second column is at a lower pressure so that higher pressure nitrogen can be used to boil lower pressure oxygen due to the fact that the boiling point of nitrogen at the higher pressure is higher than the boiling point of oxygen at the lower pressure.
By the use of such a double column air separation process, feed air is separated into components with good energy efficiency and good product purity.
However, such a process requires that the products come out of the separation at relatively low pressure. This is a drawback if one desires product at elevated pressure. For example, oxygen at elevated pressure is generally required for such applications as coal conversion to synthetic fuels and metal ore refining.
Production of elevated pressure oxygen is generally accomplished by compressing the product oxygen from the lower pressure column to the desired pressure. However, such a procedure is costly both in terms of capital costs and in operating costs to run the compressor. Furthermore, such compression has further disadvantages due to the risk of oxygen supported fire in malfunctioning compression equipment. Oxygen gas compression requires special safety considerations and equipment.
Another method which is employed to produce oxygen at elevated pressure is to withdraw oxygen as liquid from the lower pressure column and to pump the liquid oxygen to a higher pressure. The oxygen is then vaporized to produce elevated pressure oxygen gas. This method satisfactorily addresses some of the safety concerns which arise with respect to compressing oxygen gas. However, such liquid pumping processes are costly from both an equipment and operating cost standpoint.
It is desirable to have a process which allows one to employ a conventional double column air separation plant and also enables one to produce oxygen gas at a pressure greater than that of the lower pressure column without need for compressing the oxygen gas or liquid from the lower pressure column.
It is therefore an object of this invention to provide an improved double column cryogenic distillative air separation process.
It is another object of this invention to provide an improved double column cryogenic distillative air separation process wherein oxygen gas is produced at a pressure exceeding that of the lower pressure column without need for compressing oxygen gas from the lower pressure column or for pumping oxygen liquid from the lower pressure column to a higher pressure.